Remove ForceSemverUpgradeConstraints feature gate and implementation

[joelanford]

We added this feature gate very early in operator-controller's development as a shortcut to generate upgrade edges based on server, while ignoring the legacy upgrade edges (replaces/skip/skipRange). Now that operator-controller understands the legacy upgrade edges, there is no real reason to ever use the ForceSemverUpgradeConstraints feature gate because it ignores extension author intent.

Let's remove the feature gate and the code that it enables.